[dev.cc] cmd/new5a etc, cmd/internal/asm: initial C → Go conversion

This is the raw output of c2go. It needs fixes to make it compile.
Rather than make c2go do a 100% conversion (like we're doing for
liblink and the Go compilers), since this is so trivial I'm going to
make the remaining changes by hand in a followup CL.
This CL makes the next CL's diffs useful.

Also copy unmodified .y files (5a/a.y → new5a/a.y and so on)

The converted 6a/lex.c has been written to new6a/lex.go
but also to internal/asm/asm.go, because I'm going to factor
out some common code rather than convert it four times.

Change-Id: I01d5dfd6a9be3ef6191581560bdddd0ac0e8bc58
Reviewed-on: https://go-review.googlesource.com/3142Reviewed-by: Rob Pike <r@golang.org>